There's test kits available, for what you're asking the most readily available kit is an "API Master Test Kit", costs around £20.
An API test kits will cost you around £7, the master kit includes 4 tests. They last forever, I've had my tests since September and still plenty left.
What Kimberly is saying about testing is that with guppies being hardy fish its not crucial as long as you are looking out for the signs that they are struggling with ammonia levels rising.

The air pump isn't too crucial, if the filter is disturbing the water surface it should be enough to keep the oxygen level right.